Light is just one form of electromagnetic radiation, but it is the form to which the eye is sensitive. That humans and other animals respond to light is no accident, since cycles of light and dark have occurred over the course of 5 billion years of evolution. These cycles, and the mere presence of light as a medium for sensation, have shaped virtually every aspect of our psychology, from the times of day at which we are con- scious to the way we choose mating partners (using visual appearance as a cue).
Indeed, light is so useful for tracking prey, avoiding predators, and “checking out” potential mates that a structure resembling the eye has apparently evolved indepen- dently over 40 times in different organisms (Feral, 1996). Other forms of electromag- netic radiation, to which humans are blind, include infrared, ultraviolet, radio, and X-ray radiation.
Electromagnetic energy travels in repeating, rhythmic waves of differ- ent frequencies. Different forms of radiation have waves of different lengths, or wavelengths. Their particles oscillate more or less frequently, that is, with higher or lower frequency. Some of these wavelengths, such as gamma rays, are as short or shorter than the diameter of an atom; others are quite long, such as radio waves, which may oscillate once in a mile. Wavelengths are measured in nanometers (nm), or billionths of a meter (Figure 4.3). The receptors in the human eye are tuned to detect only a very restricted portion of the electromagnetic spectrum, from roughly 400 to 700 nm. This span represents the colors that are in the rainbow: red, orange, yellow, green, blue, indigo, and violet. Other organisms are sensitive to different regions of the spectrum. For example, many insects (such as ants and bees) and some vertebrate animals (such as iguanas and some bird species) see ultraviolet light (Alberts, 1989; Goldsmith, 1994). The physical dimension of wavelength translates into the psycho- logical dimension of color, just as the physical intensity of light is related to the subjec- tive sensation of brightness. Light is a useful form of energy to sense for a number of reasons (see Sekuler & Blake, 1994). Like other forms of electromagnetic radiation, light travels very quickly (186,000 miles, or roughly 300,000 kilometers, per second), so sighted organisms can see things almost immediately after they happen. Because light also travels in straight lines, it preserves the geometric organization of the objects it illuminates; the image an object casts on the retina resembles its actual structure.
wavelength the distance over which a wave of
energy completes a full oscillation
MAkING CONNECTIONS
Nearsightedness and farsightedness result when the lens of the eye focuses light rays ei- ther in front of or behind the retina. A person who is nearsighted has more difficulty viewing distant than near objects because the images are being projected in front of the retina. A farsighted person sees distant objects better than those that are close because the image is being focused behind the retina rather than on the retina. As people age, the lens loses its elasticity and its ability to accommodate, so the likelihood of becoming farsighted and needing reading glasses increases (Chapter 13).
VISION 117
Perhaps most importantly, light interacts with the molecules on the surface of many objects and is either absorbed or reflected. The light that is reflected reaches the eyes and creates a visual pattern. Objects that reflect a lot of light appear bright, whereas those that absorb much of the light that hits them appear dark.
The Eye
Two basic processes occur in the eyes (Figure 4.4). First, the cornea, pupil, and lens focus light on the retina. Next, the retina transduces this visual image into neural im- pulses that are relayed to and interpreted by the brain.
FOCUSING LIGhT Light enters the eye through the cornea, a tough, transparent tis-
sue covering the front of the eyeball. Underwater, people cannot see clearly because the cornea is constructed to bend (or refract) light rays traveling through air, not water. That is why a diving mask allows clearer vision: It puts a layer of air between the water and the cornea. An unhealthy cornea will distort light and blur vision. Corneal trans- plants, often used to treat diseased corneas, previously involved replacing the entire cornea, a procedure known as penetrating keratoplasty (PK). However, technological advances now allow only a small, thin portion of the cornea to be replaced during transplant through a procedure known as Descemet’s Stripping Endothelial Kerato- plasty (or DSEK) (Lee et al., 2009). Whereas with the older procedures, the new cornea was held in place with stitches, an air bubble holds the cornea in place with DSEK.
From the cornea, light passes through a chamber of fluid called aqueous humor, which supplies oxygen and other nutrients to the cornea and lens. Unlike blood, which performs this function in other parts of the body, the aqueous humor is a clear fluid, allowing light to pass through it. Next, light travels through the pupil, an open- ing in the center of the iris. Muscle fibers in the iris cause the pupil to expand (dilate) or constrict to regulate the amount of light entering the eye.
cornea the tough, transparent tissue covering
the front of the eyeball
pupil the opening in the center of the iris that
constricts or dilates to regulate the amount of light entering the eye
iris the ring of pigmented tissue that gives the eye its blue, green, or brown color; its muscle fibers cause the pupil to constrict or dilate.
FIGURE 4.3 The electromagnetic spectrum. Humans sense only a small portion of the electro- magnetic spectrum (enlarged in the figure), light. Light at different wavelengths is experienced as different colors. 700 Wavelength in nanometers Visible spectrum (White light) 600 550 500 450 400 650 Red Yellow Green Violet Gamma rays X rays Ultra- violet rays Infra- red rays Radar TV, FM radio Short
wave radioAM circuitsAC
1017 1015 1013 1011 109 107 105 103 10 10–1 10–3 10–5
Wavelength (in nanometers)
FIGURE 4.4 Anatomy of the human eye. The cornea, pupil, and lens focus a pattern of light onto the retina, which then transduces the retinal image into neural signals carried to the brain by the optic nerve.
Vitreous humor Cornea Pupil Iris Lens Retina Fovea Optic nerve Blind spot Aqueous humor Light kowa_c04_107-161hr.indd 117 9/13/10 10:50 AM
The next step in focusing light occurs in the lens, an elastic, disk-shaped structure about the size of a lima bean that is involved in focusing the eyes. Muscles attached to cells surrounding the lens alter its shape to focus on objects at various distances. The lens flattens for distant objects and becomes more rounded or spherical for closer objects, a process known as accommodation. The light is then projected through the vitreous humor (a clear, gelatinous liquid) onto the retina. The retina receives a constant flow of images as people turn their heads and eyes or move through space. ThE RETINA The eye is like a camera, insofar as it has an opening to adjust the amount of incoming light, a lens to focus the light, and the equivalent of photosensi- tive film—the retina. (The analogy is incomplete, of course, because the eye, unlike a camera, works best when it is moving.) The retina translates light energy from illumi- nated objects into neural impulses, transforming a pattern of light reflected off objects into psychologically meaningful information.
Structure of the Retina The retina is a multilayered structure about as thick as a sheet of paper (Figure 4.5). The innermost layer (at the back of the retina) contains two types of light receptors, or photoreceptors (photo is from the Greek word for “light”), called rods and cones, which were named for their distinctive shapes. Each retina contains approximately 120 million rods and 8 million cones.
When a rod or cone absorbs light energy, it generates an electrical signal, stimu- lating the neighboring bipolar cells. These cells combine the information from many receptors and produce graded potentials on ganglion cells, which integrate infor- mation from multiple bipolar cells. The long axons of these ganglion cells bundle together to form the optic nerve, which carries visual information to the brain.
The central region of the retina, the fovea, is most sensitive to small detail, so vi- sion is sharpest for stimuli directly at this site on the retina. In contrast, the blind spot (or optic disk), the point on the retina where the ganglion cell axons leave the eyes, has no receptor cells.
People are generally unaware of their blind spots for several reasons. Different images usually fall on the blind spots of the two eyes, so one eye sees what the other does not. In addition, the eyes are always moving, providing information about the missing area. To avoid perceiving an empty visual space, the brain also automatically uses visual information from the rest of the retina to fill in the gap. (To see the effects of the blind spot in action, see Figure 4.6.)
In some instances, retinal detachment may occur. Retinal detachment is an eye injury that results when the retina detaches from its surrounding supporting layers.
lens the disk-shaped elastic structure of the eye that focuses light
accommodation the changes in the shape of
the lens that focus light rays
retina the light-sensitive layer of tissue at
the back of the eye that transforms light into neural impulses
rods one of two types of photoreceptors; allow vision in dim light
cones one of two types of photoreceptors,
which are specialized for color vision and allow perception of fine detail
bipolar cells neurons in the retina that
combine information from many receptors and excite ganglion cells
ganglion cells nerve cells in the retina that integrate information from multiple bipolar cells, the axons of which bundle together to form the optic nerve
optic nerve the bundle of axons of ganglion
cells that carries information from the retina to the brain
fovea the central region of the retina, where
light is most directly focused by the lens
blind spot the point on the retina where the
optic nerve leaves the eye and which contains no receptor cells Ganglion cell axons Ganglion cells Bipolar cells Rod Cone
FIGURE 4.5 The retina. Light passes through layers of neurons to reach photoreceptors, called rods and cones, which respond to different wave- lengths of light. These receptors in turn connect to bipolar cells, which pass information to the ganglion cells, whose axons form the optic nerve. The photo shows rods and cones magnified thou- sands of times, along with bipolar cells.
VISION 119
Pneumatic retinopexy has become a prominent treatment method for detached reti- nas in recent years. In this procedure, a gas bubble is injected into the vitreous cavity of the eye and the patient is positioned so that the bubble closes the retinal break, much like holding down a postage stamp. The head positioning is important because the patient must be able to maintain a certain head position for several days following the surgery to allow for the break to close (Chan et al., 2008; Tornambe et al., 2002). Rods and Cones Rods and cones have distinct functions. Rods are more sensitive to light than cones, allowing vision in dim light. Rods produce visual sensations only in black, white, and gray. Cones are, evolutionarily speaking, a more recent develop- ment than rods and respond to color as well as black and white. They require more light to be activated, however, which is why we humans see little or no color in dim light. Nocturnal animals such as owls have mostly rods, whereas animals that sleep at night (including most other birds) have mostly cones (Schiffman, 1996).
Rods and cones also differ in their distribution on the retina and in their connec- tions to bipolar cells. Cones are concentrated in the fovea and decrease in density with increasing distance from the retina. Thus, in bright light, we can see an object best if we look at it directly, focusing the image on the fovea. Rods are concentrated off the center of the retina. Thus, in dim light, objects are seen most clearly by looking slightly away from them. (You can test this yourself tonight by looking at the stars. Fix your eyes directly on a bright star and then focus your gaze slightly off to the side of it. The star will appear brighter when the image is cast away from the fovea.)
Transforming Light into Sight Both rods and cones contain photosensitive pig- ments that change chemical structure in response to light (Rushton, 1962). This pro- cess is called bleaching because the pigment breaks down when exposed to light and the photoreceptors lose their characteristic color. When photoreceptors bleach, they create graded potentials in the bipolar cells connected to them, which may then fire. Bleaching must be reversed before a photoreceptor is restored to full sensitivity. Pig- ment regeneration takes time, which is why people often have to feel their way around the seats when entering a dark theater on a bright day.
Adjusting to a dimly illuminated setting is called dark adaptation. The cones adapt relatively quickly, usually within about 5 minutes, depending on the duration and in- tensity of light to which the eye was previously exposed. Rods, in contrast, take about 15 minutes to adapt. Because they are especially useful in dim light, vision may remain less than optimal in the theater for some time. Light adaptation, the process of adjust- ing to bright light after exposure to darkness, is much faster; readapting to bright sunlight upon leaving a theater takes only about a minute.
Receptive Fields Once the rods and cones have responded to patterns of light, the nervous system must somehow convert these patterns into a neural code to allow the brain to reconstruct the scene. This is truly a remarkable process: Waves of light reflected off, say, your friend’s face, pass through the eye to the rods and cones of the